Sensors that are used in the exhaust gas of a combustion engine must not only be high-temperature stable, but rather they must typically also be regulated to maintain a determined operating temperature, because both the temperature of the exhaust gas as well as the exhaust gas throughput are dependent on the operating state of the engine and vary strongly. Such sensors are operated at several hundred degrees Celsius. A typical example is the λ-sonde which can be operated at temperatures up to 1000° C.
Other types of planar exhaust gas sensors, which are presently being produced by various manufacturers, consist of a structure as is shown in FIGS. 1a, 1b and 1c in various perspectives. In this context, FIG. 1a shows the top side of the sensor as a plan view, FIG. 1b shows the sensor in a side view on the section location marked with a dashed line, and FIG. 1c shows the bottom side of the sensor in a plan view. For orientation, a coordinate system with an x-, y and z-axis is drawn in. The Figures show an elongated rectangular carrier 1, also called a transducer, which generally consists of an electrically insulating substrate. A heating layer 8 is applied to the underside 5 as shown in FIGS. 1b and 1c. This heating layer 9 comprises a heating conductor path 6 and a supply line part 2. The heating conductor path 6 is located on the sensor bottom side under the functional layer 4, which is arranged on the sensor top side 7. The functional layer 4 determines the special characteristics of the sensor, such as, the selectivity for a certain gas or the like. Then, an electrode structure 3 adapted to the special requirements of a gas sensor is applied on the sensor top side 7 under the functional layer 4. A temperature that is constant over the location must prevail on the sensor tip 10 on the sensor top side 7, in the area in which the functional layer 4 is applied. This constant temperature is achieved with the aid of the heating layer 8 and a temperature sensor or feeler, which is not shown in this illustration but is located on the sensor bottom side. Thereby the functional layer 4 is regulated to a determined temperature, the so-called operating temperature.
A further function of the elongated appearing carrier is to ensure that the temperature on the side facing away from the sensor tip 10, the so-called sensor connection side 9, is so low that synthetic plastic insulated cables can be applied as measuring lines or as power supply lines on the end of the supply line part 2 of the heating layer 8.
For the functioning of the sensor, it is of decisive significance, how constant the temperature profile is on and over the functional layer 4, and how exactly the operating temperature can be regulated.
In the application example, the heating conductor path 6 is arranged as a heating meander. The uniform zig-zag shaped meander band runs parallel to the y-axis. The constant height A of the meander here corresponds to the length L of the functional layer 4 lying thereover. The width b of the heating conductor path 6 is constant. The two ends of the heating conductor path 6 are connected with the supply line part 2 of the heating layer 8. The supply line part 2 of the heating layer 8 is guided to the sensor connection side 9.
In the EP 0,720,018 A1, a heating layer for an exhaust gas sensor is disclosed, in which the heating conductor path 6 is arranged in a serpentine shaped manner. The spacing distance of the serpentines among each other is always the same. This form similarly corresponds to a uniformly modulating meander band that runs parallel to the y-axis of the sensor.
In the U.S. Pat. No. 5,430,428, DE 43 24 659 C1 and DE 198 30 709, similarly, forms for the extending path or progression of the heating conductor path in an exhaust gas sensor are disclosed. In this context, the heating conductor path is arranged in a meandering shape. However, the uniformly modulating meander band is arranged rectangularly and also runs parallel to the y-axis of the sensor.
In all of these publications, the heating conductor path has the form of a uniformly modulating meander band. The height A of the meander band is constant during the entire extension or path progression.
A similar construction of various gas sensors is also described in the script “Industrial Gas Sensor Arrangements”, especially in part 4 by K. Ingrisch: “Semiconductor Gas Sensors” of the Instruction Course 22904/41.551 at the TAE Esslingen; G. Wiegleb (production); Esslingen 1997 and in the SAE-Paper 960692 by K. Ingrisch et al.: “Chemical Sensors for CO/NOx-Detection in Automotive Climate Control Systems”.
Arrangements of the heating layer 8 in high-temperature gas sensors are also known in which the heating conductor path 6 forms a meander band, which, beginning at the supply line part 2, first extends uniformly modulating on the one side parallel to the x-axis, and then extends in a straight line along the sensor tip parallel to the y-axis, and then again extends on the other side uniformly modulating parallel to the x-axis back to the supply line part 2. The width b of the heating conductor path 6 is not varied. The length L of the region in which the heating conductor path 6 is arranged, corresponds to the length L of the functional layer 4 lying thereover. Such a construction is disclosed, for example, in the DE 198 48 578 A1.
It is disadvantageous in all of the previously described arrangements, that a temperature gradient arises along the lengthwise axis x of the sensor, necessitated by the good thermal conductivity of the typically utilized Al2O3 substrate. This temperature gradient is subject to very large fluctuations. Thus, for a rated temperature of, for example, 600° C., this temperature gradient typically amounts to approximately BOC over the length L of the functional layer 4, as it is shown in FIG. 2b. In FIG. 2b, the temperature at various points on the sensor top side is illustrated.
In order to make the temperature distribution on the sensor top side more homogeneous, it is suggested in the EP 0,477,394 to build up or construct the heating conductor paths on the sensor tip in the form of a ladder, whereby the ladder pattern contains a plurality of parallel circuit-connected individual conductors, which can be arranged so that a homogeneous temperature distribution can be adjustably set over the length. In this context, both the width or the cross-section of the various heating conductor paths as well as the spacing between two heating conductor paths, which represent the spokes of the ladder formation, can vary.
It is disadvantageous in this publication, however, that due to the parallel circuit connection, the resistance of the heating conductor paths is reduced so far or so low that it is no longer possible to establish a resistance in the range of several ohms for the same specific resistance of the heating conductor path resistance (generally platinum), because otherwise the layer thickness of the structure would have to become so thin that it could no longer be produced by thick layer or thick film technology.
In the DE 195 23 301, a heating arrangement for a high-temperature metal oxide sensor is disclosed, in which a substrate is provided, on which, in addition to the two supply line parts of the heating layer, two measuring conductor paths are arranged, which are connected to the heating conductor path, and wherein one or more connection lines are secured to a location on the supply line part of the heating layer that is as far away as possible from the heating conductor path. This arrangement in four wire technology is illustrated as a substitute circuit diagram in FIG. 3. That means, that in addition to the wide supply line parts of the heating layer, two additional measuring lines are introduced, on which the voltage drop over the heating resistance of the heating conductor path is tapped or taken-off. In this arrangement, it is irrelevant how large the resistances R21 and R22 of the supply line parts of the heating layer are, because the voltage UH is directly taken-off or tapped on the heating resistance RH of the heating conductor path. Since the voltage UM is measured in a zero-current condition, no voltage will drop across the two tapping resistors RA1 and RA2. The resistance can be determined as RH=UH/I0 from the measured current I0 and the voltage UM. A simplified embodiment thereof is also known as state of the art, namely the so-called three-wire technology. If one assumes the two resistances of the supply line parts of the heating layer to be equal, then one can omit one of the two voltage taps. Then, one must only still measure the total voltage U0 and then obtains: RH=(2xU′M-U0)/I0. One measuring conductor and one connection contact are saved through this three-wire technology.
It is disadvantageous in this publication, however, that the temperature profile of the sensor is not constant over the length L in the x-direction, and thus the heating resistance of the heating conductor path is only to be regarded as an average value over the entire range L. Therefore, a regulation can similarly only be achieved very inexactly therewith. This is especially of disadvantage, if the temperature of the sensor housing changes strongly, as is the case, for example, in the exhaust gas of an automobile, because then the temperature gradient similarly strongly varies over the sensor chip, and thus RH can be allocated to no temperature of the functional layer.